Hi Noel,
With regard to borrowed chords, just say that my song is in C major. I would like to borrow a chord from the Cm scale. Let's say I have chord iii (Em) and I want to borrow III chord from the parallel C minor (Eb). Well I can simply enter this chord on the chord sheet and that's it. In Roman numeral notation this could just be distinguished as iii and III but I can't do that in BB because it is all upper case (at least I don't think so).

With flatted chords, just say I'm still in C major and want to use the Bdim chord although I want to lower the root by a semitone to use it as a Bb major chord. Again, in BB I could simply enter it as Bb, although I can make the Roman numeral notation accept and interpret correctly with the entry of bVII. I also tried this with bIII and bVI and this gives me, correctly, Eb and Ab respectively.

If I stick to Roman Numerals then some of what I wanted is already there.

I know this sounds a bit trivial because I can simply enter the equivalent chord directly. But what I'd like is a visual record of the choices I have made for the chords in song when I look at the chord sheet at a much later date and can instantly see what I meant at each chord.

Perhaps there is a way of manually entering chord types that BB will understand?